ann1087 — Announcement

The First Methane Dwarf Orbiting a Dead Star

23 November 2010

An international team of astronomers has used ESO telescopes and other facilities to discover and study a unique and exotic star system comprised of a very cool, methane-rich (or T-) dwarf star and a “dead” white dwarf star in orbit around each other. The system is a “Rosetta Stone” for T-dwarf stars, giving scientists a way of finding the mass and age of this old methane dwarf.

The team, led by Dr Avril Day-Jones of the Universidad de Chile and composed of astronomers from the University of Hertfordshire (UK), and the University of Montreal (Canada), is publishing its results in the journal Monthly Notices of the Royal Astronomical Society.

The system is the first of its type to be found. Methane is a fragile molecule destroyed at warmer temperatures, so is only seen in very cool stars and giant planets like Jupiter. Neither giant planets nor T-dwarf stars are massive enough for the hydrogen fusion that powers the Sun to take place, meaning that they simply cool and fade over time.

This binary is providing a crucial test of our knowledge of the physics of ultra-cool stellar atmospheres because the white dwarf can be used to establish the age of both objects.

The methane dwarf was identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) and the Gemini North Telescope in Hawaii was used to measure its temperature and spectrum. The white dwarf component of the pair was studied using ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile.

The two stars are separated by at least 2.5 thousand billion km today, but would have been closer in the past. The new discovery shows that despite their fragility, such binaries are able to remain united even in the maelstrom of the galactic disc.